At present, a multicast communication technology has become a hot technology in network applications due to its capability of conveying copies of the same information to a plurality of destinations. Multi-Protocol Label Switching (MPLS) technology extends Label Distribution Protocol (LDP) and Resource Reservation Protocol-Traffic Engineering (RSVP-TE) to support MPLS multicast trees. In the MPLS technology, a MPLS Label Switched Path (LSP) may be established through MPLS signaling such as LDP protocol. A packet is classified with a “Forwarding Equivalence Class (FEC)” as it enters the network, and a corresponding label is attached to the packet. The label is sent along with the packet while the packet is forwarded to its next hop. After receiving an MPLS packet, an intermediate Label switching Router (LSR) forwards the packet directly according to the label contained in the packet header without retrieving the destination node using the Internet Protocol (IP) address contained in the IP packet header.
In order to obtain a more efficient multicast transmission with simplified operations, a multicast application in LSP tunnels or multi-access links generally adopts a label assignment technology to ensure that all the LSRs at the next hop in the multicast application use the same label.
Current label assignment may adopt upstream label assignment, or upstream-suggested downstream label assignment. In the upstream label assignment, an upstream LSR in a multicast LSP assigns a common label for all the downstream LSRs at the next hop. In the downstream label assignment, the label to be assigned is suggested by the upstream LSR, and all the LSRs at the next hop assign the same label suggested by the upstream LSR.
FIG. 1 is a flowchart illustrating the signaling procedure of a downstream label assigning method according to the prior art. Referring to FIG. 1, the downstream label assigning method in the prior art includes the following steps.
In block 101, a downstream LSR 12 requests an upstream LSR 14 through a label mapping message carrying a specific label to suggest a label for the next hop, wherein the specific label indicates that the downstream LSR 12 is instructing the upstream LSR 14 to suggest a label.
In block 102, the upstream LSR 14 transmits a suggested label to the downstream LSR 12 in a label request message.
In block 103, the downstream LSR 12 returns a label mapping message to the upstream LSR 14 indicating that the label suggested by the upstream LSR is adopted.
FIG. 2 is a schematic diagram illustrating a network scenario in which unicast and multicast co-exist. As shown in FIG. 2, a unicast LSP 22 passes through router R2, router R9, router R10 and router R5 in sequence, and the label assigned to router R10 is label K. A path of a multicast LSP 24 passes through router R1, to router R9 and then to router R3 and router R7 in parallel, and the label assigned to router R3 and router R7 is also the label K.
When router R10 attempts to join the multicast LSP, all the downstream LSRs of router R9 have to use the same label, i.e. router R10 has to use the same label K as router R3 and router R7. However, since the unicast LSP exists prior to the multicast LSP and the label used by router R10 in the unicast LSP is the same label K which is to be used by router R10 in the multicast LSP, a conflict of the label K may arise when router R10 joins in the multicast. A current solution of a conflict of labels is to abort the establishment of the multicast LSP, and to establish a new unicast LSP between router R9 and router R10 to transmit the multicast data. Thus, the multicast data must be transmitted through the unicast path, which makes the procedure more complex.